Chem Problem, can't figure it out. HELP

[239493]

Ok here's the problem out of the Kaplan book:

What is the molecular weight of an unknown gas if 2.5g of it occupies 2L at 630 torr and a temperature of 600K.

Ok, so I know the formula PV=nRT.
We have V which is 2L.
We have temperature which is 600K.
We have pressure which is 630 torr but must be converted to atmosphere.
So i set up the ratio 760torr/1atm = 630torr/x and I got 1.2 atm.
So atm is 1.2.
And we know n is = g/mm, with mm being the question at hand.
So n = 2.5/x.
And R = 0.0821

So I set up this problem as the following.

(1.2atm)(2L) = 2.5g/x (0.0821)(600K)

we then get 2.4 = 123/x
solve for x and you get 51 as the molecular weight.

What i dont get is that the book says the answer is 74. I have no Idea why! and its bothering me. Can someone help me please.

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[fenixtnlfan]

Because 630 torr=0.83 atm. You did your math wrong there.

 

[239493]

how did you get 0.83, i keep gettin 1.2

 

[supraman]

 

[239493]

nvm, got it haha sorry about that

 

[fenixtnlfan]

Lol...all of us do that at some point. Just don't do it when it's important

 

[Short Round]

lol! this is a great emoticon haha!

 

[WhiteSnows]

Your pressure is calculated wrong. 630 torr/ 760torr = 0.829 atm.